For a subsea well bore, the well is provided with a wellhead placed on the seabed to ensure the sealing of the well and oil reservoir against its environment (the sea). Then, for hydrocarbon fluid production, a Christmas tree is usually fitted on the wellhead to control the flow of hydrocarbon fluid (for example, oil or gas).
Usually, a wellhead assembly is equipped at its upper end with a Christmas tree comprising a plurality of valves for securing the well and a control flow device for controlling the flow of hydrocarbon fluid pulled out from the well.
The document U.S. Pat. No. 5,992,527 discloses such a wellhead assembly having a tubing hanger adapted to suspend a tubing that extends inside the casing and inside the well. The wellhead is equipped with an in-line tree comprising valves and an horizontal tree aligned with a lateral bore of the in-line tree. The flow of hydrocarbon fluid is controlled by additional valves and equipments secured to the horizontal tree forming a huge and heavy conventional Christmas tree above the wellhead assembly.
Such wellhead equipped with a Christmas tree for controlling the hydrocarbon fluid flow, and for providing security fail safe valves are difficult to be assembled down to the seabed. Therefore, such completion extends during days, and is costly.